Abstract
A modulated-frequency photometric interferometer is described for precise calibration of vibration pickups. The plates of the Fizeau-type interferometer are subjected to sinusoidal vibrations—one plate at frequency f and amplitude ξ0, the other plate at frequency f′ and amplitude γ0, where f>4f′. A photomultiplier examines a portion of the fringe pattern and its output is sent through a bandpass filter tuned to f′. The filtered signal is proportional to J0(4πξ0/λ). Accurate determinations of ξ0 may be made by adjusting this amplitude such that J0(4πξ0/λ) arrives at one of its low-order nulls. This method is analogous to the visual fringe disappearance method currently in wide use for vibration pickup calibration which yields the same functional dependence on amplitude ξ0. The modulated frequency interferometer, however, does not suffer from the errors of human judgment, having an electrical rather than visual output. Nulls of J0(4πξ0/λ) can be located with a precision exceeding that obtainable with visual fringe disappearance.
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